A known call configuration used for sending human speech from a first mobile telephone to a second mobile telephone over a network involves tandem operation. In tandem operation, the speech is encoded at the first mobile telephone into a compressed form to save bandwidth, which is the amount of data that can be passed along a communications channel in a given period of time.
This encoded speech is then transmitted from the mobile station through the air over a Radio Frequency (RF) Interface to a first base station. A transcoder rate adapter unit (TRAU) associated with the first base station takes the encoded speech signal and decodes it into a different and decompressed format so that it can be transmitted through a fixed telephone network to a second base station.
The decompressed format used for transmission through a fixed telephone network may be generally a 64 kbps G.711 A-law or μ-law or other standard format. At this second base station, the decompressed speech must be encoded for a second time into a compressed format where it is then transmitted again through the air over an RF Interface to the second mobile station.
A second transcoder rate adapter unit associated with the second base station performs this second encoding operation. At the second mobile station this encoded compressed speech is finally decoded to audible human speech.
The drawback to this call configuration is that it requires two encoding/decoding operations, and each one of these operations degrades the quality of the final speech signal that is heard by the person using the second mobile telephone (i.e., the person listening to someone else talk over a mobile telephone).
Tandem Free Operation (TFO) eliminates this drawback by sending human speech from a first mobile telephone to a second mobile telephone over a network with a reduced the number of coding/decoding operations performed on the speech signal. In tandem free operation, the speech is encoded at the first mobile telephone and is not decoded until it is received by the second mobile telephone. Tandem free operation eliminates the need to decode the signal at the first base station into the 64 kbps format and to then encode the signal at the second base station back into compressed format.
Tandem free operation bypasses this procedure and instead uses in-band signaling to send the speech signal in its originally compressed form from the first mobile telephone to the second mobile telephone. This is accomplished by sending the compressed speech signal through the fixed telephone network as a series of tandem free operation frames (TFO frames) included as part of the in-band communication between the components involved in the operation. This eliminates a coding/decoding (collectively “codec”) operation and results in a better quality speech signal.
Existing tandem free operation protocols are not without drawbacks. In order for tandem free operation to function properly, among other things the transcoder rate adapter units on both ends of the operation must be in proper communication with each other. Transcoder rate adapter units operate in many different functional states. One of these functional states is known as the operation state, and another is known as the synchronization lost state. During the back and forth communication that occurs during speech signal transmission, these components may switch or oscillate between states. This oscillation is especially prevalent in instances of high propagation delay during communication between two TRAUs.
A problem arises when one component, such as a TRAU associated with a base station, switches into the synchronization lost state, but a TRAU associated with the another base station is functioning in the operation state. When this occurs, at least one of the TRAUs may stop sending or receiving the speech signal for a period of time. This signal distortion due to components involved in speech signal transmission oscillating between different functional states leads to time gaps in the speech that reduce the quality of speech heard at the second mobile telephone.